Espresso machines of the above mentioned type are known in the art, as described, for instance in EP-A-1 360 918, EP-A-2 314 183, EP-A-2 314 182, EP-A-2 133 011.
The invention also relates to the brewing process.
In actual use, coffee machines as described above have the drawback of a shorter time for wetting the dose of ground coffee as compared with traditional machines, whose wetting time usually ranges from 5 to 7 seconds from the time the dispensing valve is opened, which is deemed to be an optimal time in coffee machines to obtain proper dispersion of water through the dose of ground coffee.
As is known, the brewing or wetting time is the time required, from the moment the dispensing valve device is opened, for hot water to increase its pressure from a low value, generally slightly above 1 bar, to the traditional operating value of about 8-9 bar.
Coffee machines having an independent heater on each dispenser unit, which form the subject of the present invention, cannot ensure the above timings for pressure increase from the initial value of 1 bar to the operating value of 8-9 bar at which coffee is actually dispensed, due to their construction.
This is because, when the coffee dose dispensing procedure is started, the water heater on the dispenser is almost full of water and communicates with the source of pressurized water, traditionally represented as a pump, whereby it is almost instantaneously exposed to the operating pressure of the pump, which is about 8-9 bar.
Therefore, water is expelled out of the chamber of the heater, and brews the coffee in the filter holder, when it is actually already at the maximum operating pressure of 8-9 bar.
The object of the present invention is to obviate the above described drawback by providing a technical arrangement that allows the machines equipped with a heater on the dispenser unit to also ensure adjustable times for wetting the dose of ground coffee in the filter holder, and corresponding to the times that have been found in the art to be optimal, from the time the dispensing valve device is opened.